The present invention relates generally to paper making machines and, more particularly, to apparatus for use in a drying section of a paper machine for preventing conditions which tend to deteriorate the contact between the web and a supporting fabric.
Specifically, the invention relates to apparatus for preventing the generation of positive pressures in the narrowing gaps defined by the surfaces of drying cylinders and the web supporting fabric as it approaches and/or moves away therefrom. The apparatus generally comprises a blow box which extends substantially over the entire width of the web and which is adapted to be connected to a source of blow air. The blow box is provided with at least two blow air discharge apertures, such as nozzle slots or the like, transverse to the direction of run, i.e., the longitudinal direction, of the web.
The running speeds of paper machines have in recent years been constantly increasing and are now approaching 1,500 meters per minute. The tendencies of the web to flutter and become loosened from supporting fabrics have therefore become serious problems in the operation of modern paper machines.
The present invention has particular applicability in connection with paper machine drying sections using so-called single-wire draws. In such drying sections a web in contact with a supporting fabric runs from a drying cylinder in one line of drying cylinders to a drying cylinder in another line of drying cylinders so that on the cylinders of one line the web is situated between the supporting fabric, i.e., the drying wire, and the surface of the cylinder, while on the cylinders of the other line the web is situated on the outside with the drying wire situated between the cylinder surface and the web. Such single-wire draw arrangements are advantageous in that the web is supported by the drying wire at all times without any open draw, or at least without any substantially lengthy open draw, thereby reducing the risks of wrinkles being formed in the web and of web breakage.
It is well known that a thin boundary layer of air follows the moving surfaces of the web supporting fabric with those particles of air which are in contact with the moving surface moving at the same speed as the fabric.
Narrowing gaps or throats are defined in single-wire draw drying sections by the surfaces of the drying cylinders or rolls and the web supporting fabric or wire. Such narrowing gaps are termed inlet gaps in the case where the web carrying fabric moves into or enters such a gap, and are termed outlet gaps when the web supporting fabric moves away from the gap.
If all of the surfaces defining the narrowing gap are substantially impervious to air, an air flow is produced between the boundary air layers, both in inlet and outlet gaps, in a direction opposite to the direction of the boundary air layer flows. Therefore, a positive pressure exists across the fabric in an inlet gap due to the dam-like effect of the boundary layer air flows. On the other hand a negative pressure exists across the fabric in outlet gaps due to the suction created by the boundary layer air flows.
When air pervious fabrics, such as wires, are used as is conventional, the pressure differential across the web supporting fabric, caused by the boundary layer air flows, usually causes a detrimental air flow through the fabric. Indeed, several so-called pocket ventilation devices have been proposed in the past which are based on the pumping effect of the open drying wires described above.
It is also conventional to provide the first and second drying groups of a paper machine with a single-wire draw, which is often arranged so that the paper web is situated between the wire and the cylinder on those cylinders of the upper line, while the drying wire is situated on the drying cylinder surface with the web situated over the wire on the drying cylinders of the lower line.
The operation of single-wire draws in drying sections of the type described above has given rise to numerous problems and drawbacks which directly result from the differences in pressure caused by the boundary layer flow at the different sides of the web supporting wire. Air tends to flow from the higher pressure side to the lower pressure side through the wire and thereby tends to loosen the support contact between the wire and the paper web. In particular, such problems arise in the regions of the narrowing inlet and outlet gaps, in both of which the pressure differential across the wire tends to detach the paper web from the supporting wire. If such detachment occurs, the paper web will behave in an unstable manner due to the air flows which occur in the pockets defined between the drying cylinders. A web that has become loosened from its supporting wire is often lengthened on the lower drying cylinders under the effects of centrifugal force, which in turn results in wrinkling of the web in an inlet gap. In the worst case, the web can break which necessitates shutting down the operation of the entire paper machine.
In an attempt to overcome the problems described above, various sealing arrangements have been suggested whose objective is to prevent the boundary layer air flow from following the moving surfaces of the web supporting fabric and the drying cylinder into the narrowing gap defined thereby. For example, such a sealing arrangement is disclosed in patent application DE-OS No. 2,712,184. Such mechanical sealing arrangements are limited in that they cannot be located sufficiently close to the moving web or fabric due to the risk of damage to the web and the risk of abrasion to the wire. For example, it is not possible to seal a narrowing outlet gap by mechanical means of this type since moving paper web requires a distance of at least 10 mm from the sealing member. Although it is theoretically possible to situate a mechanical seal closer to the fabric in the case of a narrowing inlet gap, such a provision only shifts the point at which the pressure differential exists to a location ahead or upsteam of the seal where a positive pressure caused by the damming effect of the boundary layer flow is then produced.
Conventional mechanical seal arrangements also have the drawback that the efficiency of these seals deteriorates over the course of time if the seals are located so close to the surfaces that abrasion takes place. In patent application FI No. 803891, assigned to the assignee of the instant application, a method and apparatus are suggested for preventing the formation of positive and negative pressures which would be detrimental to the supporting contact between the fabric and the web. The device disclosed in that application is situated proximate to the initial line of contact between the surfaces of the drying cylinder and the web supporting wire. This device improves the support contact between the wire and the web in a region from the initial line of contact (or ultimate line of contact) to a region situated about 200 to 300 mm from the line of initial or ultimate contact between the wire and the web which is, as a rule, sufficient in a group of drying cylinders of modern paper machines.
However, there may be free draws between the rolls or cylinders of two drying groups which are so long that the range of 200 to 300 mm of improved support contact achieved by the apparatus disclosed in application FI No. 803891 is not sufficient, whereby the web tends to become loosened from the supporting wire or fabric and flutters.
Indeed, in single-wire draw drying sections, the support contact between the web and the wire is not deteriorated solely by the positive pressures generated in the narrowing gaps or throats, the effects of which extend only over a relatively short distance. Rather, the contact between the web and the supporting fabric or wire is also deteriorated due to the instability and fluttering of the run of the web supporting fabric which occurs over the relatively long, free runs thereof between drying cylinders of the upper and lower lines of drying cylinders.